In a conventional clip, that is a clip with the trademark name "GEM", a wire is rolled into rectangular form in one plane and is formed with two arms to the inner and outer sides. Such a clip has many disadvantages in that it does not tightly grip thick sheets without sliding. That is to say, at the time that thick sheets are gripped between the two arms, the long arm opens and the edge side of the sheets slip from the side face. Also, the gripping power concentrates at the edge sides of the sheets. For these reasons, the part which opens up in use is easily caught in other objects, and the clip comes off easily some times and it is inconvenient for stacking purposes. The stress concentrates at the small back part which is located at the base of the two arms, and therefore the stress of such part exceeds the elastic limit of the wire and does not return to its initial condition. Also, at the time of turning of gripped pages, it is impossible to turn up pages enough because the edge of the crossing arms disturbs the turn-up pages. The clip is pushed out from the sheets when the gripped sheets open a little strongly. Thus, such a GEM clip is not suitable for thick sheets.
At the present time, as a clip unit for thick sheets, a double clip is known which has a gripper with an isosceles triangle and a horizontal section is formed having a constant breadth. Such a double clip is used for sheets which have less height than the back part of the double clip because the thickness for gripping is limited according to the height of the back part which corresponds to the base of the isosceles triangle of the gripper. Therefore, in most cases, the high back part of the gripper sticks out from the edge of the gripped sheets at a right angle toward the gripped face. This stuck-out part is inconvenient at the time that the gripped sheets are piled up. A lever also interferes with the turn-up of pages. Such double clip further has a defect in that it is expensive to produce as compared to other kinds of clips.
A clip providing crossing arms which cross at the back part of the gripped part is disclosed in Japanese Utility Model No. 366551 and an all-round clip is disclosed in Japanese Utility Model No. Showa 47-35853. These clips provide for capacity in that the gripper tightly grips both faces of the gripped sheets even at the time of gripping thick sheets. However, in the former, the direction of the touching line is such that the part which has the main gripping effect faces closely at a right angle toward the edge of the gripped sheets and is very long. Also, in the latter, one side of the gripped part has a wide face. Consequently, a long touching line of the gripped part has a direction which slips easily on the part which is formed by the touching face is hardly wide enough for good gripping. Therefore, there is a defect in that the gripped sheets easily slip off, or the clip slides out easily from the gripped sheets when in use. Also, there are problems in the efficiency, the cost and the handling because these clips need to be united from a plurality of parts.